1. Field of the Invention
The present invention relates to automatic dishwashing detergent compositions and, more particularly, to liquid automatic dishwashing detergent compositions which are thixotropic and have improved cup retention ability.
2. Description of the Background
Powdered, automatic dishwasher detergents suffer from several disadvantages such as non-uniform composition, a tendency to cake resulting in the formation of lumps which are difficult to disperse, dustiness (and the associated complications for users with allergies), and costly manufacturing techniques.
Automatic dishwasher detergent liquids (ADL's), while they overcome many of the problems noted above with respect to powdered automatic dishwasher detergents, suffer from their own disadvantages. In particular, one of the more serious problems encountered in the use of ADL's is that of poor cup retention. Cup retention refers to the tendency of the ADL to remain in a closed dispensing cup inside the dishwasher during the pre-wash cycle. Most automatic dishwashers have two dispensing cups for dishwasher detergent. The first cup is generally an open cup, whose contents are immediately contacted with the wash water upon initiation of the wash cycle. The second cup is closed in some fashion and is opened automatically at a desirable point later in the wash cycle. In most existing automatic dishwashers, the closure mechanism on the second cup has been designed for powdered formulations, and this mechanism often lacks a seal sufficient to contain a highly fluid formulation. Moreover, even when a seal is present, in older model dishwashers the seal mechanism is often worn. Consequently, the performance of many ADL formulations in dishwashers present in many consumers' homes has been heavily influenced by the rheological properties which control cup retention. Where these rheological properties are not adequately designed, most of the ADL is lost in the first wash cycle which results in inadequate washing during the second wash cycle when normally the ADL contained in the second cup would be dispensed. The further complication of this premature leaking of the ADL out of the dispensing cup is that residual surfactant which is intended to carry over into the rinse cycle as a rinse aid is lost during the first wash cycle. Thus, the consumer experiences inadequate cleaning due to the lack of detergent during the second wash cycle and spotting of glassware because of insufficient residual concentrations of rinse aid during the rinse cycle.
Manufacturers of ADL's have attempted to design the rheology of the formulation to provide a gel-like consistency when quiescent to improve cup retention. To this end, natural clays have typically been used at levels of about 3% by weight in order to provide the rheology required. The clays which are employed, e.g., the smectite clays, including montmorillonite (bentonite), are costly, especially when obtained in a pure, low color form. Further, since these clays are mined from naturally occuring deposits, there is also a problem with uniformity of available materials.
Aqueous dispersions of boehmite alumina have been used as thickeners in various formulations. However, it has not been proposed to use alumina as a thickener in ADL's since it was believed that the other components present in ADL's inhibit the thickening behavior of the alumina dispersion.
British Patent No. 2,176,495 discloses a thixotropic dishwasher detergent composition which employs clay or a similar material as a thixotropic thickener and the polyvalent metal salt of a long chain fatty acid as an additive to provide physical stability in terms of resistance to phase separation and settling.
It would be clearly desirable to provide an ADL which utilized less expensive, chemically uniform alumina of high whiteness as a thickener rather than more expensive, offcolor clays currently used.